Metal tantalum is a valve metal. It can form a layer of dense oxide film on its surface to have a property of unilateral conduction. An anode film made of tantalum has chemical stability (especially in acidic electrolyte), high resistivity (7.5×1010 Ω·cm), high dielectric constant (27.6) and small leakage current. Furthermore, the anode film has advantages including a broad range of working temperature (−80 to 200° C.), high reliability, anti-vibration and long service life. It is an ideal material for preparing tantalum capacitor with small volume and high reliability.
Tantalum capacitor is an electronic device with tantalum as metal anode, having a dielectric oxide film which is directly formed on the surface of tantalum through the oxidation of the anode. Tantalum powder has a very high specific surface area. The specific surface area remains high even after pressing and sintering due to its characteristic pore structure, and thereby the resulting capacitor has high specific capacitance.
Among the methods for preparing tantalum powder, the reduction method of potassium fluotantalate with sodium is the most widely used and the most maturely developed preparation method worldwide.
The reduction method of potassium fluotantalate with sodium is a method in which tantalum powder of capacitor grade is prepared, using K2TaF7 and Na as main feedstocks, and using a halide salt such as NaCl, KCl or a mixture of halide salts as diluent, with the following main reaction mechanism:K2TaF7+5Na=Ta+5NaF+2KF  (1)
The above reaction occurs between K2TaF7 and liquid sodium, in the nitrogen atmosphere and at certain temperature. The tantalum powder resulting from the reduction is water washed and acid washed, and is then heat-treated. Subsequently the powder is reduced and deoxidized with magnesium, and the final tantalum powder with high purity is obtained.
Currently tantalum powder of capacitor grade is developing in a direction of high specific capacitance and high purity. It is well known that the specific capacitance of tantalum powder is in proportion to its specific surface area. In other words, the smaller the mean particle diameter of tantalum powder is, the higher the specific surface area is, and the higher the specific capacitance is. The key technology to achieve high specific capacitance of tantalum powder is the preparation of tantalum powder with smaller mean particle diameter. In respect to the reduction method of potassium fluotantalate with sodium, the core of the development is to control the formation, the distribution and the growth of the crystal nucleus during the reduction with sodium by controlling the reduction conditions including the compositions of potassium fluotantalate and diluent dissolving salts, the reduction temperature, the rate of injection of sodium etc., in order to prepare the tantalum powder having certain specific surface area and particle diameter. A mechanical method is to obtain tantalum powder with finer particles by controlling the conditions of hydrogenation milling or ball milling. The reduction of halide with hydrogen uses preparation technology of nanoscale powder, and the particle size of the resulting tantalum powder is in nanoscale with very high specific surface area.
Doping is an important technical means to achieve high specific capacitance of tantalum powder, and thus is generally used in the production and the development of tantalum powder with high specific capacitance. The main purposes of doping during the preparation method of tantalum powder are: 1) to refine the tantalum powder; and 2) to inhibit the growth of grains of the tantalum powder, to the most possibly maintain higher specific surface area of the tantalum powder, and to reduce loss of specific capacitance of the tantalum powder. Doping can be conducted during various methods. Doped elements commonly used include N, Si, P, B, C, S, Al, O etc. and compounds thereof. Doped elements generally segregate at the surfaces of grain-boundaries, reacting with tantalum at high temperature to form various compounds of tantalum. Doping comprises not only incorporating one element in one step, but also doping multiple elements in multiple steps. By this way, tantalum powder can be refined, and at the same time loss of specific capacitance of tantalum powder can be reduced.
It is a widespread operation to dope nitrogen in tantalum powder in the tantalum powder industry, especially in the production of tantalum powder with high specific capacitance.
U.S. Pat. No. 6,876,542 provides a production method of nitrogen-containing metal powder, and porous sintered body and solid electrolytic capacitor using the metal powder. The nitrogen containing metal powder has a ratio W/S between the nitrogen content W (ppm) and the specific surface area S (m2/g) as measured by a BET method, that falls within a range from 500 to 3000. The patent further provides a method in which nitrogen is introduced during the reduction to increase nitrogen content in tantalum powder.
U.S. Pat. No. 7,066,975 provides nitrogen-containing metal powder, production method thereof, and porous sintered body and solid electrolytic capacitor using the metal powder. The patent provides nitrogen-containing metal powder which is a solid solution containing 50-20,000 ppm nitrogen, in which the metal is particularly tantalum, preferably containing P, B, O or a combination thereof. It is characterized that nitrogen is in solid solution form, and the mean particle diameter of the nitrogen-containing metal powder is in a range of 80 to 360 nm.
U.S. Pat. No. 7,473,294 provides nitrogen-containing metal powder, production method thereof, and porous sintered body and solid electrolytic capacitor using the metal powder. The patent provides nitrogen-containing metal powder (having surface layer and inner layer) which is a solid solution containing 50-20,000 ppm nitrogen. The nitrogen is in solid solution form. The metal is tantalum. The nitrogen uniformly permeates from the surface layer of the metal to the inner layer. And the particle diameter of the metal powder is ≦250 nm.
U.S. Pat. No. 6,432,161 provides another nitrogen-containing metal powder, production method thereof, and porous sintered body and solid electrolytic capacitor using the metal powder. The patent provides a production method of nitrogen-containing metal powder, comprising a compound of niobium or tantalum is reduced with a reducing agent, and nitrogen is simultaneously incorporated into the reaction system, and nitrogen-containing niobium or tantalum powder in solid solution form is formed, wherein the nitrogen is simultaneously incorporated in niobium or tantalum powder. The patent is an alternative to the reduction method of potassium fluotantalate with sodium, which is suitable for the reduction doping nitrogen of other compounds of niobium or tantalum.
Chinese patent application CN1498144A relates to a preparation method of sintered particles. The particles is made of a mixture of refractory metals and refractory metal nitrides. It is found that the particles have higher proportion of intra-aggregate pores as than those singly made of refractory metals or nitrides of refractory metals. There is provided improved powder of capacitor grade, anode and capacitor made thereby. Where the mixture contains 50 to 75 w/w % of refractory metal nitrides, the porosity and the total intrusion volume of the particles are maximized. The total pore surface area of the particles is 50% higher than that of single refractory metal nitrides. A substrate consisting of a powder mixture of 50/50 or 25/75 w/w % refractory metals/refractory metal nitrides generates solid capacitor with higher recovery rate of capacitor and lower ESR. To be brief, tantalum and tantalum nitride or niobium and niobium nitride are directly mixed, and are shaped by pressing, to form anode of capacitor.
Accordingly, the methods for doping nitrogen in tantalum powder used in prior art focus on the incorporation of nitrogen-containing gas into the reaction system. The nitrogen is present in tantalum powder in solid solution form. However, such methods for doping nitrogen is less effective, and tantalum powder with high nitrogen content cannot be obtain, and the amount of doped nitrogen cannot be accurately controlled. Therefore, there is still need for a method for doping nitrogen in tantalum powder having good effectiveness and controllability.